- Phytoplasmas
Definition
Phytoplasmas are wall-less prokaryotic pathogens restricted to the phloem tissue of plants and transmitted by phloem-feeding insects like leafhoppers, psyllids, and planthoppers. They are associated with several plant diseases, particularly those involving stunted growth, yellowing, and witches’ broom symptoms.
Characteristics
- Morphology: Pleomorphic (variable shape) due to the absence of a rigid cell wall.
- Size: Small (0.2–0.8 µm in diameter).
- Genome: Highly reduced, around 530–1350 kb, making them dependent on their hosts.
- Habitat: Found in the phloem tissue of infected plants and the bodies of insect vectors.
- Transmission:
- Spread by phloem-feeding insects like leafhoppers and psyllids.
- Can also be transmitted through grafting, infected cuttings, or vegetative propagation.
- Symptoms:
- Yellowing (chlorosis)
- Witches’ broom (proliferation of shoots)
- Stunting
- Floral abnormalities (e.g., phyllody—conversion of flowers into leaf-like structures)
Examples of Diseases Caused by Phytoplasmas
- Grassy Shoot Disease of Sugarcane: Caused by Candidatus Phytoplasma sacchari.
- Little Leaf of Brinjal: Caused by Candidatus Phytoplasma aurantifolia.
- Coconut Lethal Yellowing: Caused by Candidatus Phytoplasma palmae.
- Aster Yellows: Caused by Candidatus Phytoplasma asteris.
- Sesame Phyllody: Caused by Candidatus Phytoplasma sesami.
- Spiroplasmas
Definition
Spiroplasmas are wall-less, helical-shaped prokaryotes found in phloem tissue of plants and the hemolymph (blood) of their insect vectors. They are distinct due to their spiral or helical morphology and motility.
Characteristics
- Morphology: Helical or spiral-shaped and motile.
- Size: Small (0.2–0.3 µm wide and up to 5 µm long).
- Genome: Larger than phytoplasmas, approximately 1 Mb.
- Habitat: Found in the phloem of plants and hemolymph of insect vectors.
- Transmission:
- Transmitted by leafhoppers and planthoppers.
- Can be experimentally transmitted through mechanical inoculation.
- Symptoms:
- Stunting
- Yellowing (chlorosis)
- Reduced vigor and sometimes wilting
Examples of Diseases Caused by Spiroplasmas
- Citrus Stubborn Disease: Caused by Spiroplasma citri.
- Corn Stunt Disease: Caused by Spiroplasma kunkelii.
- Periwinkle Disease: Caused by Spiroplasma phoeniceum.
Comparison Between Phytoplasmas and Spiroplasmas
Feature | Phytoplasmas | Spiroplasmas |
Morphology | Pleomorphic | Helical or spiral-shaped |
Size | 0.2–0.8 µm | 0.2–5 µm |
Cell Wall | Absent | Absent |
Genome Size | Smaller (530–1350 kb) | Larger (~1 Mb) |
Habitat | Phloem of plants | Phloem of plants and hemolymph of insects |
Transmission | Phloem-feeding insects (e.g., leafhoppers, psyllids) | Phloem-feeding insects (e.g., leafhoppers) |
Examples of Diseases | Little leaf of brinjal, Grassy shoot of sugarcane | Citrus stubborn, Corn stunt |
